Singlet-triplet transition in a quantum dot: effect on mesoscopic transport

The T=0 transport properties of a wire interacting with a lateral two-level quantum dot are studied by using an exact numerical calculation. The wire conductance, the spin–spin correlation and the Kondo temperature are obtained as a function of the dot level energy spacing. When the dot has two electrons and spin S_D1, the wire current is totally quenched by the S=1 Kondo effect. The Kondo temperature is maximum at the singlet–triplet transition and its dependence upon the dot energy spacing follows a non-universal scaling law.     

XOR gate response in a mesoscopic ring with embedded quantum dots

We address XOR gate response in a mesoscopic ring threaded by a magnetic flux . The ring, composed of identical quantum dots, is symmetrically attached to two semi-infinite one-dimensional metallic electrodes and two gate voltages, viz, V_a and V_b, are applied, respectively, in each arm of the ring which are treated as the two inputs of the XOR gate. The calculations are based on the tight-binding model and the Green’s function method, which numerically compute the conductance–energy and current–voltage characteristics as functions of the ring-electrodes coupling strengths, magnetic flux and gate voltages. Quite interestingly it is observed that, for =₀/2 (₀=ch/e, the elementary flux-quantum) a high output current (1) (in the logical sense) appears if one, and only one, of the inputs to the gate is high (1), while if both inputs are low (0) or both are high (1), a low output current (0) appears. It clearly demonstrates the XOR behavior and this aspect may be utilized in designing the electronic logic gate.     

Impurity related optical properties in tuned quantum dot/ring systems

In this paper, we explore the linear, third-order nonlinear, and total optical absorption coefficient (OAC) and refractive index change coefficient (RICC) of a GaAs doped quantum dot/quantum ring (QD/QR) with parabolic-inverse squared potential in conjunction with modified Gaussian confinement and taking into account the presence of on-centre shallow donor and or acceptor impurity. Calculations are done via the compact density matrix formalism and the iterative method. The two-dimensional parabolic QD/QR is subjected to uniform magnetic field oriented perpendicularly to the plane of the structure. The energy levels and wave function are derived within the framework of effective-mass and parabolic band approximation. The results exhibit that the OACs and RICC are clearly affected by different parameters of the applied confinement, strength of magnetic field, and the presence of impurity. The variation of confinement potential, nature of impurity, dot radius, cyclotron frequency of the parabolic confinement potential, and geometric parameter of the on-centre repulsive potential lead to either a red-shift or a blue-shift of the resonant peaks of the OACs and of the maximum and minimum of the RICC together with significant variations of the magnitudes of these resonant structures.     

NOR gate response in a double quantum ring: An exact result

NOR gate response in a double quantum ring, where each ring is threaded by a magnetic flux , is investigated. The double quantum ring is sandwiched symmetrically between two semi-infinite one-dimensional metallic electrodes, and two gate voltages, namely, V_a and V_b, are applied, respectively, in lower arms of the two rings those are treated as the two inputs of the NOR gate. A simple tight-binding model is used to describe the system, and all the calculations are done through the Green’s function formalism. Here we calculate exactly the conductance–energy and current–voltage characteristics as functions of the ring-to-electrode coupling strengths, magnetic flux and gate voltages. Our numerical study predicts that, for a typical value of the magnetic flux =₀/2 (₀=ch/e, the elementary flux-quantum), a high output current (1) (in the logical sense) appears if both the inputs to the gate are low (0), while if one or both are high (1), a low output current (0) results. It clearly demonstrates the NOR gate behavior, and this aspect may be utilized in designing an electronic logic gate.     

Electron transport in a double quantum ring: Evidence of an AND gate

We explore AND gate response in a double quantum ring where each ring is threaded by a magnetic flux ?. The double quantum ring is attached symmetrically to two semi-infinite one-dimensional metallic electrodes and two gate voltages, namely, Va and Vb, are applied, respectively, in the lower arms of the two rings which are treated as two inputs of the AND gate. The system is described in the tight-binding framework and the calculations are done using the Green's function formalism. Here we numerically compute the conductance-energy and current-voltage characteristics as functions of the ring-to-electrode coupling strengths, magnetic flux and gate voltages. Our study suggests that, for a typical value of the magnetic flux ?=?₀/2 (?₀=ch/e, the elementary flux-quantum) a high output current (1) (in the logical sense) appears only if both the two inputs to the gate are high (1), while if neither or only one input to the gate is high (1), a low output current (0) results. It clearly demonstrates the AND gate behavior and this aspect may be utilized in designing an electronic logic gate.     

Signatures of a noise-induced quantum phase transition in a mesoscopic metal ring

We study a mesoscopic ring with an inline quantum dot threaded by an Aharonov-Bohm flux. Zero-point fluctuations of the electromagnetic environment capacitively coupled to the ring, with omega(s) spectral density, can suppress tunneling through the dot, resulting in a quantum phase transition from an unpolarized to a polarized phase. We show that robust signatures of such a transition can be found in the response of the persistent current in the ring to the external flux as well as to the bias between the dot and the arm. Particular attention is paid to the experimentally relevant cases of Ohmic (s = 1) and sub-Ohmic (s = 1/2) noise.     

Feasibility study of the quantum XOR gate based on coupled asymmetric semiconductor quantum dots

We propose an implementation of the quantum XOR (controlled-NOT) gate on the basis of coupledasymmetricquantum dots. Results of our numerical simulations show that the coupling constant of the dipole–dipole interaction and the probability of spontaneous emission can be tuned over a wide range by a proper choice of the potential profile, material parameters, and distances between the dots. We argue that the use of the asymmetric potential profile provides better conditions for having the Ising-type interaction between the dots than earlier proposed schemes based on regular symmetric quantum dots biased with an electric field. Our system gives better resolution of different quantum states, avoids any undesirable time evolution of these states, and can be driven with a femtosecond laser. The qubit manipulation and time coherency requirements are also discussed.     

Control of Aharonov–Bohm oscillations in a AlGaAs/GaAs ring by asymmetric and symmetric gate biasing

The control of the Aharonov–Bohm effect on a AlGaAs/GaAs ring structure is studied by employing two in-plane-gates. By applying a gate voltage to one of the gates, a change of the oscillation pattern due to the additional potential induced in one branch of the ring is observed. The change of the oscillation frequency as well as the phase is attributed to the multi-channel transport. In case of a symmetric biasing, where both gates are biased simultaneously, a larger voltage is required to change the oscillation pattern than for the case when only one gate is used. This effect is explained by a partial compensation of the phase difference between both branches of the ring.     

Operating a geometric quantum gate by external controllable parameters

A scheme to perfectly preserve an initial qubit state in geometric quantum computation is proposed for a single-qubit geometric quantum gate in a nuclear magnetic resonance system. At first, by adjusting some magnetic field parameters, one can let the dynamic phase be proportional to the geometric phase. Then, by controlling the azimuthal angle in the initial state, we may realize a geometric quantum gate whose fidelity is equal to one under cyclic evolution. This means that the quantum information is no distortion in the process of geometric quantum computation.     

Buckyball quantum computer: realization of a quantum gate

We have studied a system composed by two endohedral fullerene molecules. We have found that this system can be used as good candidate for the realization of quantum gates. Each of these molecules encapsules an atom carrying a spin, therefore they interact through the spin dipole interaction. We show that a phase gate can be realized if we apply static and time dependent magnetic fields on each encased spin. We have evaluated the operational time of a π-phase gate, which is of the order of ns. We made a comparison between the theoretical estimation of the gate time and the experimental decoherence time for each spin. The comparison shows that the spin relaxation time is much larger than the π-gate operational time. Therefore, this indicates that, during the decoherence time, it is possible to perform some thousands of quantum computational operations. Moreover, through the study of concurrence, we get very good results for the entanglement degree of the two-qubit system. This finding opens a new avenue for the realization of quantum computers.     

Inductive current in a quantum ring

The induction current in a single-channel quantum ring placed in a quasi-stationary magnetic field is calculated taking into account the scattering of electrons by phonons.     

Spin effects in a quantum ring

Recent experiments that are reviewed explore the spin states of a ring-shaped many-electron quantum dot. Coulomb-blockade spectroscopy is used to access the spin degree of freedom. The Zeeman effect observed for states with successive electron number allows to select possible sequences of spin ground states of the ring. Spin-paired orbital levels can be identified by probing their response to magnetic fields normal to the plane of the ring and electric fields caused by suitable gate voltages. This narrows down the choice of ground-state spin sequences. A gate-controlled singlet–triplet transition is identified and the size of the exchange interaction matrix element is determined.     

Nonvolatile gate effect in a ferroelectric-semiconductor quantum well

Field effect transistors with ferroelectric gates would make ideal rewritable nonvolatile memories were it not for the severe problems in integrating the ferroelectric oxide directly on the semiconductor channel. We propose a powerful way to avoid these problems using a gate material that is ferroelectric and semiconducting simultaneously. First, ferroelectricity in semiconductor (Cd,Zn)Te films is proven and studied using modified piezoforce scanning probe microscopy. Then, a rewritable field effect device is demonstrated by local poling of the (Cd,Zn)Te layer of a (Cd,Zn)Te/CdTe quantum well, provoking a reversible, nonvolatile change in the resistance of the 2D electron gas. The results point to a potential new family of nanoscale one-transistor memories.     

Remarkable enhancement of terahertz conductivity of graphene tuned by periodic gate voltage

Optical conductivity of a graphene sheet subject to a one-dimensional cosinusoidal potential is studied. We find that the optical conductivity can be tuned by the strength of the superlattice potential. When this strength is within a certain range the terahertz conductivity can be enhanced by two orders of magnitude.     

Micromechanically tuned ring resonator in silicon on insulator

We demonstrate a monolithically integrated micromechano-optical device where the resonance wavelength of a silicon ring resonator is tuned by perturbing the evanescent field with an electrostatically actuated silicon nitride microcantilever. The resonance wavelength can be tuned over 125 pm.     

Metal-insulator transition in a system of superconducting vortices caused by a metallic gate

We address a recent experiment in which a strong decrease of the resistance of a superconducting film has been observed when a remote unbiased gate was placed above the film. Here we explain the experimental finding as a suppression of the vortex tunneling due to the orthogonality catastrophe of the electrons inside the gate. We interpret the change in the resistance of the film as a "metal-insulator" transition in the system of vortices induced by the gate.     

氢化杂质和厚度效应对高斯势量子点中二能级体系量子跃迁的影响

在计及氢化杂质和厚度效应下,分别选取抛物线型限定势阱和高斯函数型限定势阱描写盘型量子点中电子的横向限定势和纵向限定势,采用Lee-Low-Pines-Pekar变分法推导出量子点中电子的基态和第一激发态能量本征值和本征函数,以此为基础,构造了一个二能级结构,并基于二能级体系理论,讨论了电子在磁场作用下的量子跃迁.结果表明,高斯函数型限定势比抛物线型限定势更能精准反映量子点中真实的限定势;量子点的厚度对电子的跃迁概率的影响不凡;电声耦合强度、介电常数比、磁场的回旋频率、高斯函数型限定势阱的阱深和阱宽等对电子基态与第一激发态声子平均数、能量以及量子跃迁的影响显著.     

Experimental teleportation of a quantum controlled-NOT gate

Teleportation of quantum gates is a critical step for the implementation of quantum networking and teleportation-based models of quantum computation. We report an experimental demonstration of teleportation of the prototypical quantum controlled-NOT (CNOT) gate. Assisted with linear optical manipulations, photon entanglement produced from parametric down-conversion, and postselection from the coincidence measurements, we teleport the quantum CNOT gate from acting on local qubits to acting on remote qubits. The quality of the quantum gate teleportation is characterized through the method of quantum process tomography, with an average fidelity of 0.84 demonstrated for the teleported gate.     

Optimizing a phase gate using quantum interference

A controlled interference is proposed to reduce, by two orders of magnitude, the decoherence of a quantum gate for which the gate fidelity is limited by coupling to states other than the /0> and /1> qubit states. This phenomenon is demonstrated in an ultracold neutral atom implementation of a phase gate using qubits based on motional states in individual wells of an optical lattice.